The structure and function of DNA are fundamental concepts in molecular biology, primarily centered around nucleotides, which are the building blocks of DNA. Each nucleotide consists of a sugar, a phosphate group, and a nitrogenous base. There are four nitrogenous bases in DNA: adenine (A), cytosine (C), guanine (G), and thymine (T). These bases are categorized into two groups based on their structure: purines, which have two carbon rings (adenine and guanine), and pyrimidines, which have one carbon ring (cytosine and thymine).
Base pairing is governed by Chargaff's rules, which state that adenine pairs with thymine (A-T) through two hydrogen bonds, while cytosine pairs with guanine (C-G) through three hydrogen bonds. This specific pairing is crucial for the stability of the DNA structure, as it ensures that the bases align correctly, forming what are known as base pairs.
The DNA molecule has a distinct directionality, with each strand having a 5' (five prime) end and a 3' (three prime) end. The backbone of DNA is formed by alternating sugar and phosphate groups, where the 5' phosphate group of one nucleotide binds to the 3' hydroxyl group of the next. This orientation is essential for DNA replication and function.
In terms of structure, DNA typically exists as a double helix, where two strands wind around each other, with the sugar-phosphate backbones on the outside and the base pairs on the inside. Each complete turn of the helix contains approximately 10 base pairs and spans about 3.4 nanometers in length, resulting in a distance of 0.34 nanometers per nucleotide.
The strands of DNA are complementary, meaning that the sequence of bases on one strand determines the sequence on the other. This complementary nature also leads to the anti-parallel arrangement of the two strands, where one strand runs in the 5' to 3' direction and the other runs in the 3' to 5' direction.
Additionally, the DNA double helix features major and minor grooves, which are important for protein binding and interaction. While the B-form of DNA is the most common and well-studied, there are also A and Z forms of DNA. The A-form is shorter and more compact, while the Z-form is a left-handed helix, with its significance still being explored. Understanding these variations in DNA structure is crucial for comprehending its diverse roles in biological processes.
